A composite part may be bonded or cured in an oven or an autoclave where heat is applied to the part while supported on a cure tool that maintains the shape of the part during the curing process. Techniques have been developed for curing composite parts without the need for an oven or autoclave. However, these techniques have been limited to curing relatively small, simple parts and/or require relatively complicated and/or expensive tooling. Recently, curing of relatively small composite parts has been achieved using induction heating equipment employing ferromagnetic susceptors that produce a maximum, constant temperature when inductively heated. For example, heating blankets using inductively heated susceptors have been used to cure relatively small areas of a composite rework patch applied to a structure such as an aircraft skin.
In certain known heating blankets, the blankets are constructed by threading springs of susceptor wire onto a length of a conductor wire that is designed for carrying high frequency current, commonly referred to in the art as a Litz wire. When threading the susceptor wire onto the conductor wire, it is generally desired to orient the susceptor wire as near to perpendicular as possible to the direction of current flow in the Litz wire. A near perpendicular orientation is desired so as to maximize the induced magnetic fields into the susceptor wire which creates heat by virtue of eddy currents created by the wire. By using springs (i.e., pre-formed or wrapped onto the Litz wire), the susceptor can be oriented along the Litz wire in order to capitalize on a high density of susceptor per unit length of the Litz wire and keep the susceptor wire in the region of highest magnetic field strength (i.e., as close to orthogonal to the direction of current flow within the Litz wire).
This threaded spring configuration has been shown to produce suitable results for certain heating blanket applications, but also has demonstrated certain limitations. For example, in such spring configurations, a large amount of Litz wire is typically required to carry the appropriate amount of current for large heating blankets. In addition, a large amount of Litz wire is typically also required to maintain an applied voltage within certain safety levels, and also to produce the required amount of heat. Therefore, the spring threaded configurations do not lend themselves to providing a practical heating blanket for large heating or curing applications. Moreover, is has been proven difficult to keep the susceptor springs from tangling with one another within the heating blanket. In addition, susceptor springs were not cost effective for large sized heating blankets.
Accordingly, there is a need for cost effective methods and devices that can be utilized to fabricate susceptor based heating blankets while customizing such blankets so as to achieve desired heating profiles, especially for heating large composite structures.